The invention relates to a method of manufacturing a broadband cholesteric polarizer, in which a liquid-crystalline, cholesterically ordered layer comprising reactive chiral monomers and reactive nematogenic monomers of different reactivity is polymerized by exposure to radiation. The invention also relates to devices for carrying out the method in accordance with the invention.
Broadband cholesteric polarizers and methods of manufacturing same are known per se, for example, from EP-A 0606939 U.S. Pat. No. 5,721,603 EP-A 0606940 U.S. Pat. No. 5,506,704 and WO 96/02016, U.S. Pat. No. 5,737,044. By means of cholesteric polarizers it is possible to convert unpolarized light to circularly polarized light in a substantially loss-free manner. Polarizers of this type comprise a thin layer of a cholesterically (i.e. chirally nematically) ordered material. This material contains chiral, liquid-crystalline molecules having such a structure that they order themselves more or less spontaneously into a spiral-shaped or helical structure. The pitch of this helix can be increased by adding a quantity of a non-chiral, liquid-crystalline (i.e. nematogenic) material to the chiral, liquid-crystalline material. The exact pitch is governed by the ratio between the quantities of chiral and non-chiral liquid-crystalline molecules as well as by their chemical structure.
If this material is provided in the form of a thin layer on a substrate or between two substrates, the helical structure assumes such an orientation that the axis of the helix extends transversely to the layer. Such a layer is capable of reflecting a narrow band of light whose wavelength corresponds to the product of the pitch and the refractive index of the material and whose direction of polarization corresponds to the handedness of the helical structure. By virtue of this property, a cholesteric layer can very suitably be used in an optical polarizer. It is noted that the expression "the refractive index" of a material is to be understood to mean in this context the geometric mean (n.sub.e +n.sub.o)/2 of the ordinary refractive index n.sub.o and the extraordinary refractive index n.sub.e of this material.
Broadband cholesteric polarizers are distinguished from the customary cholesteric polarizers by the presence of a relatively broad reflection band. The bandwidth of the customary cholesteric polarizers is only approximately 40-50 nm. In the case of broadband polarizers, bandwidths of 100 nm, 150 nm, 200 nm and even more than 400 nm have been achieved. It is noted that the band position of a cholesteric filter is defined as the center of the wavelength range in which the reflection takes place. A width of a band is defined as the difference in wavelength between the long-wave and the short-wave edge positions of the band. The wavelength of an edge position is defined as the wavelength at which the intensity amounts to 50% of the maximum intensity.
EP 606940 describes an elegant method of manufacturing a broadband cholesteric polarizer. Use is made of a mixture comprising reactive chiral monomers and reactive nematogenic monomers, which exhibit a different reactivity. For the reactive monomers use can be made of compounds containing a reactive group on the basis of acrylates, epoxy compounds, vinylethers and thiolene systems, as described, inter alia, in U.S. Pat. No. 5,188,760. Monomers containing different reactive groups generally exhibit a different reactivity. A difference in reactivity also occurs if one type of monomers contains one reactive group and the other type of monomers contains two (identical) reactive groups.
A layer of this mixture is polymerized by means of (actinic) radiation, in particular UV radiation. In this process, the conditions are selected in such a manner that during the polymerization operation a radiation profile of varied intensity is formed in the layer. As a result, diffusion processes take place in the cholesteric layer during polymerization. This leads to a variation in the composition of the helical structure, so that the pitch, viewed across the thickness of the layer, varies within certain limits. As a result, this cholesteric polarizer exhibits a relatively broad reflection band.
It has been found that the method described in EP-A 606940 can be improved. The Applicant has experimentally established that small fluctuations, for example in the radiation gradient or in the UV intensity, can strongly influence the diffusion processes of the reactive monomers. This may lead to relatively large differences in the bandwidth of the cholesteric polarizers manufactured by means of said known method. Therefore said known method should be improved, in particular, with respect to the reproducible manufacture of polarizers having a correct position of one of the two edges of the reflection band. This applies, for example, to polarizers as described in EP-A 95203209.2 U.S. Pat. No. 5,825,444.